1. Field of the Invention
This invention relates to a viewfinder system adapted for a photographic camera, a video camera or the like, and more particularly to a viewfinder system arranged such that, when a viewer (camera operator) views through an eyepiece lens a viewing plane (focal plane) on which a viewfinder image (object image) is formed, the viewfinder system detects a visual line of the viewer, so that shooting actions such as a distance measuring action (automatic focusing), a light measuring action, etc., are performed by using information on the visual line detected.
2. Description of Related Art
Visual-line detecting devices for detecting which point on a viewing plane a viewer is viewing, i.e., for detecting a visual line (a visual axis) of the viewer, have heretofore been variously proposed. For example, in Japanese Laid-Open Patent Application No. SHO 61-172552, there is disclosed a device in which a parallel light beam is projected from a light source onto the anterior segment of the eye to be detected and a visual axis (fixation point) is found by using a cornea-reflected image (Purkinje image) resulting from light reflected from a cornea and a position where an image of a pupil is formed.
FIG. 1 is a diagram for explaining the principle of the conventional visual-line detecting method.
Referring to FIG. 1, a light source 25 such as a light emitting diode is disposed on the focal plane of a light projection lens 27 to project an insensible infrared light onto the viewer. The infrared light emitted from the light source 25 is made into parallel rays of light by the light projection lens 27. The parallel rays of light are reflected by a half mirror 22 to illuminate the cornea 21 of the eyeball 20 of the viewer. Then, a cornea-reflected image (Purkinje image) d resulting from a part of the infrared light reflected by the surface of the cornea 21 passes through the half mirror 22 and is condensed by a visual line lens 24 to form again an image d' on an image sensor 26.
Light beams from the end parts a and b of the iris 23 are guided through the half mirror 22 and the visual line lens 24 to the surface of the image sensor 26 to form images a' and b' of the end parts a and b of the iris 23. If the rotation angle .theta. of the optical axis AX2 of the eyeball 20 relative to the optical axis AX1 of the visual line lens 24 is small, with the Z-coordinates of the end parts a and b of the iris 23 assumed to be Za and Zb, respectively, the Z-coordinate Zc of the center c of the iris 23 can be expressed as follows: ##EQU1##
Further, with the Z-coordinate of the position where the cornea-reflected image d is generated assumed to be Zd and a distance from the center of curvature o of the cornea 21 to the center c of the iris 23 to be oc, the rotation angle .theta. of the optical axis AX2 of the eyeball 20 approximately satisfies the following relation formula: EQU oc.multidot.sin .theta..apprxeq.Zc-Zd
Therefore, the rotation angle .theta. of the optical axis AX2 of the eyeball 20 can be obtained by detecting the positions of the singular points (the cornea-reflected image d and the end parts a and b of the iris 23) projected on the image sensor 26. For this purpose, the formula (a) is changed to the following formula (b): ##EQU2##
In the formula (b), .beta. represents a magnification determined by a distance L between the generated position of the cornea-reflected image d and the visual line lens 24 and a distance L.sub.0 between the visual line lens 24 and the image sensor 26. The distance L.sub.0 can be obtained in some suitable manner.
With a visual-line detecting device applied to the viewfinder system of, for example, a photographic camera, a video camera or the like, in order to accurately detect the visual line of the viewer (camera operator) who is looking into the viewfinder system, it is important to have an image which is reflected from the eyeball (eyeball image) accurately formed (with a high degree of resolution) on the image sensor. Generally, the eyeball image can be accurately formed on the image sensor by arranging each of an image forming system (a visual-line detecting optical system) and an eyepiece lens to be composed of a plurality of lenses. That arrangement, however, causes the whole optical system to become complex and large. Then, it becomes difficult to place such a large and complex optical system inside a small limited space available within a camera or the like.